crystals Article On the New Oxyarsenides Eu5Zn2As5O and Eu5Cd2As5O Gregory M. Darone 1,2, Sviatoslav A. Baranets 1,* and Svilen Bobev 1,* 1 Department of Chemistry and Biochemistry, University of Delaware, Newark, DE 19716, USA; [email protected] 2 The Charter School of Wilmington, Wilmington, DE 19807, USA * Correspondence: [email protected] (S.B.); [email protected] (S.A.B.); Tel.: +1-302-831-8720 (S.B. & S.A.B.) Received: 20 May 2020; Accepted: 1 June 2020; Published: 3 June 2020 Abstract: The new quaternary phases Eu5Zn2As5O and Eu5Cd2As5O have been synthesized by metal flux reactions and their structures have been established through single-crystal X-ray diffraction. Both compounds crystallize in the centrosymmetric space group Cmcm (No. 63, Z = 4; Pearson symbol oC52), with unit cell parameters a = 4.3457(11) Å, b = 20.897(5) Å, c = 13.571(3) Å; and a = 4.4597(9) Å, b = 21.112(4) Å, c = 13.848(3) Å, for Eu5Zn2As5O and Eu5Cd2As5O, respectively. The crystal structures include one-dimensional double-strands of corner-shared MAs4 tetrahedra (M = Zn, Cd) and As–As bonds that connect the tetrahedra to form pentagonal channels. Four of the five Eu atoms fill the space between the pentagonal channels and one Eu atom is contained within the channels. An isolated oxide anion O2– is located in a tetrahedral hole formed by four Eu cations. Applying the valence rules and the Zintl concept to rationalize the chemical bonding in Eu5M2As5O(M = Zn, Cd) reveals that the valence electrons can be counted as follows: 5 [Eu2+] + 2 [M2+] + 3 × × × [As3–] + 2 [As2–] + O2–, which suggests an electron-deficient configuration. The presumed h+ × hole is confirmed by electronic band structure calculations, where a fully optimized bonding will be attained if an additional valence electron is added to move the Fermi level up to a narrow band gap (Eu5Zn2As5O) or pseudo-gap (Eu5Cd2As5O). In order to achieve such a formal charge balance, and hence, narrow-gap semiconducting behavior in Eu5M2As5O(M = Zn, Cd), europium is theorized to be in a mixed-valent Eu2+/ Eu3+ state. Keywords: oxypnictide; Zintl phases; europium; mixed valence; zinc; arsenic; oxygen 1. Introduction Zintl phases are salt-like intermetallic compounds in which electron transfer from the less electronegative elements to the more electronegative elements occurs. Often, the electron donors are alkali, alkaline earth, or rare earth metals, which become cations, while the electron acceptors are typically late d-block metals or p-block metalloids, which become anions. The latter form polyanionic subunits which feature polar covalent bonding [1–3]. Zintl phases are typically semiconductors; however, variations in bonding within the polyanionic substructure and changes in elemental composition can result in varying electrical properties. The complex bonding patterns and diverse electronic properties found in Zintl phases make them interesting subjects in areas of colossal magnetoresistance, mixed valency, ferromagnetism and antiferromagnetism, and thermoelectricity [4–6]. Within the realm of the “classic” Zintl phases, pnictides are the compounds that are characterized by the most polar covalent bonding due to the relatively high electronegativities of the group 15 elements, and as such, they possess semiconducting behavior with widely varied band gaps. There are also pnictides that can be considered as extensions of the Zintl concept, as in the case of the BaFe2As2 Crystals 2020, 10, 475; doi:10.3390/cryst10060475 www.mdpi.com/journal/crystals Crystals 2020, 10, 475 2 of 13 family of compounds, where superconductivity with a critical temperature of nearly 40 K [7] can be achieved. Other pnictogen-containing Zintl phases have become the gold standard for materials with ultra-low thermal conductivity, which when coupled with favorable charge transport enables very high values of the thermoelectric figure of merit ZT [8–10]. These interesting properties are related to the complex bonding arrangements that can be realized based on smaller anionic subunits which connect to form a variety of polyanionic motifs—from 1D chains and ribbons, to 2D layers and even into extended 3D frameworks. Over the past 15 years, our research group has systematically explored numerous alkaline earth metal and rare earth metal pnictide systems. The synthesized compounds have almost exclusively been examples of “classic” Zintl phases, which despite featuring complex crystal and electronic structures, have nevertheless shown excellent adherence to the Zintl–Klemm rules [11–17]. While oxides are typical examples of ionic solids and can hardly be classified as Zintl phases, various oxypnictides fall under the latter category, displaying covalent bonding with a much higher degree of polarity than found in typical intermetallic compounds. Oxypnictides have garnered increased interest in recent years due to the discovery of superconductivity at relatively high transition temperatures, TC,[18–20]. Many examples have already been studied, and systematic investigations of element substitution and doping have yielded higher and higher transition temperatures, with materials like Sm0.98Th0.2FeAsO0.775F0.225 reaching TC of 58.6 K [21]. Although oxides are not an area of focus for our research group, it occasionally happens that a new oxygen-bearing compound is serendipitously found as a side product in a reaction intended to produce another phase. This is especially true for experiments involving the very reactive alkali and alkaline earth metals, as well as Eu from the rare-earth metal block. A brief exposure to the atmosphere during the sealing of an ampoule or inadvertent reduction of alumina (crucible material) might be all the opportunity necessary for unintended oxygen to find its way into the reaction materials or products, resulting in an unplanned oxide phase. While they are infrequent discoveries in our lab, these uncommon oxygen-stabilized phases have been documented in several previous studies, such as Ca4Bi2O, Ba2Cd3–δBi3O, Ba5Cd2Sb5Ox (0.5 < x < 0.7), Eu5Cd2Sb5O, and Ba5Cd2Sb4O2 [22–26]. This small group is expanded with the two title compounds presented herein. In this paper, the synthetic routes of the title compounds will be described, along with the single-crystal X-ray diffraction and structure determination. A review of the key structural features, such as polyanionic subunit building blocks and connections, and comparisons of bond distances with other relevant compounds, follows. Trends in unit cell dimensions and bond distances with elemental substitution are explored, as well as a rationalization of the structure as a Zintl phase confirmed with electronic structure calculations. 2. Materials and Methods 2.1. Synthesis The handling of the starting materials was performed inside an argon-filled glove-box and under vacuum. Eu, Zn, Cd, As, and Pb were purchased from Alfa Aesar or Sigma-Aldrich with stated purity greater than 99.9% and were used as received. Neither oxygen nor another oxygenated material was deliberately used. Single crystals of Eu5Zn2As5O and Eu5Cd2As5O were obtained as minor side products of reactions of Eu, M, and As (M = Zn or Cd) using the Pb-metal flux method. For Eu5Cd2As5O, Eu, Cd, and As were measured in an elemental ratio of 22:4:18, respectively, and combined in an alumina crucible with an excess of Pb (ca. 5 based on molar Eu content) and sealed in an evacuated fused silica jacket. The × ampoule was heated in a programmable furnace from 100 to 1000 ◦C at a rate of 200 ◦C/hour, allowed to homogenize at this temperature for 10 h, then cooled to 900 ◦C at a rate of 2 ◦C/hour, allowed to dwell at this temperature for 72 h, and then cooled to 600 ◦C at a rate of 100 ◦C/hour. At 600 ◦C, the ampoule was removed from the furnace, inverted, and spun in a centrifuge for 30 s to separate the Pb Crystals 2020, 10, 475 3 of 13 from the crystals. After that, the ampoule was brought back into the glove-box and cracked open. The obtained crystals were isolated and studied by X-ray diffraction (vide infra). As can be suggested from the loaded ratio, the target phase we aimed to synthesize was Eu21Cd4As18, isostructural to Eu21Cd4Sb18 [27]. However, the majority phase identified for this reaction was the ternary compound, Eu2CdAs2 [28], in the form of very small crystals, with Eu5Cd2As5O occurring as a few distinctly different bar-shaped crystals. A second reaction conducted with an elemental ratio of Eu, Cd, As, and Pb of 26:4:18:100, respectively, using a maximum reaction temperature of 850 ◦C produced polycrystalline Eu14CdAs11 as the majority phase [29], again with Eu5Cd2As5O being present as just 2-3 isolated bar-like crystals, which were easily identifiable by sight and subsequently confirmed by single-crystal X-ray diffraction methods. For Eu5Zn2As5O, a slightly different synthetic route was taken, despite the nominal composition being similar. Elemental ratios of 3:1:3 for Eu, Zn, and As, respectively, were measured and combined in an alumina crucible with an excess of Pb (ca. 5 based on molar Eu content) and sealed in an × evacuated fused silica jacket. The ampoule was heated in a programmable furnace from 100 to 1000 ◦C at a rate of 100 ◦C/hour, allowed to homogenize at this temperature for 24 h, then cooled to 650 ◦C at a rate of 3 ◦C/hour. At 650 ◦C, the ampoule was removed from the furnace, inverted, and spun in a centrifuge for 30 s to separate the Pb from the crystals. The major phase identified for this reaction was Eu11Zn6As12 [30], which was obtained as large needle-shaped crystals, with Eu5Zn2As5O occurring as a few smaller crystals with habits akin to those of the previously described Eu5Cd2As5O. No systematic efforts were made to synthesize either Eu5Zn2As5O or Eu5Cd2As5O in bulk quantities. Apparently, the reported quaternaries were encountered in our reactions by serendipity. A partial oxidation of the starting materials, most likely the Eu metal, is speculated to be the source of oxygen.